Electric utility power generating systems generally comprise an alternating current electric power generator driven by a turbine. While some systems employ water turbines, most systems utilize steam turbines in which a controlled steam flow through the turbine regulates the rotational velocity of a driven turbine shaft. The steam flow is controlled, in response to electric power demands placed on the generator, such that the frequency of the alternating current produced by the generator is maintained at a constant value regardless of variations in electric power demands placed on the generator. The steam flow in turn is controlled by various flow control valves such as throttling valves and steam bypass valves.
With increases in the size and capacity of steam turbines, the sizes of throttling valves and bypass valves have also increased in order to handle larger volumetric flows of steam pressures to turbines. Furthermore, light water nuclear power generating systems, which operate at lower steam pressures and temperatures than fossil fuel turbines, require a much greater volumetric steam flow in order to obtain a desired power output. The advent of nuclear powered turbines and the increased size of turbines generally have necessitated that the sizes of turbine steam valves be increased substantially. For example, a suitable size for the valve seat diameter of a main control valve used in an exemplary fossile fuel turbine is approximately seven inches, while the diameter of an equivalent valve seat for a valve in a nuclear powered turbine may be as large as twenty inches. The size of bypass valves also increases considerably in nuclear powered steam generators. Because the size and weight of a valve design must be increased substantially in order to handle the large volumetric steam flow rates found in nuclear generating systems, former designs for control valves and bypass valves which have been suitable for fossil fuel systems are not easily adapted to handle the greater capacity. For example, some applications require valves to perform over large pressure drops ranging down to 2 percent of the supply steam pressure and the valves must also perform efficiently when modulated over small pressure drops, e.g., when a valve is nearly wide open.
The transition to larger valve designs has introduced unusual noise and vibration problems when controlling high steam flow rates. For example, U.S. Pat. No. 3,602,261 to Brown, assigned to Westinghouse Electric Corp., and incorporated herein by reference, discloses a flow muffler which greatly reduces severe vibrations and intense noise levels which occur during the throttling of high capacity steam control valves. However, while the flow muffler is effective in reducing vibration induced noise, it is now believed that other, more subtle, vibration sources exist in valve designs of this type. In particular, prior analyses and proposed remedies have focused primarily upon modifications of the flow muffler under the premise that vibrations would be eliminated by limiting vibratory excitations in and around one valve component, i.e., the muffler. It is now believed that other major factors induce vibrations in numerous areas throughout the valve structures. It is therefore desirable to provide an improved valve design suitable for high volumetric flow of pressurized fluids which has a more limited response to vibrational excitations than former designs.